1. Field of the Invention
The present invention relates to a magnetic fluid composition in which fine magnetic particles are dispersed in a solvent. More specifically, the invention relates to a magnetic fluid composition having excellent stability and which is capable of maintaining excellent fluidity for a long period of time.
2. Discussion of the Background
A magnetic fluid has conventionally been investigated for use in various kinds of devices. A magnetic fluid comprises, for example, a material prepared by coating the surface of fine magnetic particles such as magnetite, Mn--Zn ferrite, Ni--Zn ferrite, or the like, obtained by the coprecipitation method, with a higher fatty acid, or the like as a surfactant by adsorbing and dispersing the coated particles in an oily or aqueous solvent. Examples of such a magnetic fluid and method of preparation are disclosed in Japanese Patent Publications No. 4078/1978 and No. 17118/1978 and Japanese Laid-Open Patent Application No. 105093/1984, or the like.
However, in such a magnetic fluid in which an unsaturated aliphatic acid is used as a surfactant of fine magnetic particles such as magnetite, which coats the particles, and in which the particles are dispersed in an oily solvent, when the fluid is used for a long period of time, a problem which is encountered is that the magnetic fluid gradually deteriorates by oxidation or other polycondensation reactions and gels, which results in lower fluidity.
In recent years, magnetic fluids have become particularly useful in a wide range of fields. For example, they have been used as a shaft sealing solution, a damper solution, a heating medium, or the like and the use of such fluids under high temperature or high humidity conditions is gradually increasing. Under the conditions of high temperatures and high humidity, the reactivity and corrosiveness of magnetic fluids increase which lead to deterioration of the fine magnetic particles because of oxidation, whereby inherent functions are abruptly lost within a short period of time in many cases. In view of these problems, there is a high demand for a magnetic fluid which exhibits less deterioration by preventing oxidation and which can maintain excellent fluidity over a long period of time.
In general, deterioration of an oil has been prevented by adding a small amount of a phenolic, aminic or sulfur containing antioxidant. In this embodiment, it is extremely unusual to add an antioxidant in an amount exceeding 0.5% based on the amount of the oil. With regard to magnetic fluid technology, currently not much is known about what kind of solvent should be used to which an antioxidant is added, how much of what kind of an antioxidant should be used, and other such factors. Incidentally, in Japanese Laid-Open Patent Application No. 105093/1984, there is a description of the necessity of adding a phenolic, aminic, thiophosphate, or the like antioxidant. However, no specific examples of such formulations are described nor any specific method of addition of components. The document further discloses that there is no significant difference between added amounts of antioxidants of 0.1% and 10% in preventing oxidative destruction However, this is the result of ignoring the solubility of an antioxidant in the base oil solvent so that it cannot be said that these conditions give sufficient results depending on the amount of antioxidant added. Also, the effects of addition of antioxidant are not limited to the inhibition of oxidative destruction of the base oil solvent, nor the capability of the antioxidant in preventing polymerization or gelation. A need therefore continues to exist for a magnetic fluid which is very resistant to oxidative deterioration.